The present invention relates to television receivers and, more particularly, to television receivers having a picture-in-picture (PIP) feature, that is, receivers in which a small, compressed version of a video signal is displayed within a larger "main" picture.
In television receivers having a PIP feature, two channels are provided, namely, a main channel and a PIP channel. The main channel is a conventional television channel and receives an incoming composite television signal and furnishes a luminance and two color difference signals therefor. The PIP channel also receives a composite television signal, but instead processes the signal digitally. A digital, reduced version of the received picture is stored in a memory. This digital reduced version is read out from the memory, converted back to analog form, and the resulting color difference and luminance signals are combined with the corresponding signals from the main channel under control of a synchronous switching circuit. The combined signals are then converted to red, green and blue color signals for application to a display.
A vertical pre-filter is provided to process the digital television signal before it is stored in memory. Since the picture is to be displayed in a much smaller area than the main picture, sampling is required. However, sampling at less than the Nyquist frequency introduces low frequency beats. Also, the compressed signal, if not preprocessed, may be subject to line flicker and aliasing.
A U.S. Patent Application filed concurrently herewith and assigned to the Assignee of the subject application, discloses a vertical filter which carries out the low frequency filtering function reliably and with a minimum of components. However, the length of this vertical filter, that is, the number of television lines over which a weighted average can be taken, is limited to the line decimation factor. This is very restrictive in practice and compromises the quality of the resultant reduced picture.